1. Field of the Invention
The present invention relates to an image reading apparatus, particularly to an apparatus capable of detecting a foreign object that may cause an image defect when performing image reading.
2. Description of the Related Art
In the related art, there are known image reading apparatuses capable of optically reading documents, such as image scanners, digital copiers and facsimiles, which perform “moving document reading” using an automatic document feeder (ADF) in order to efficiently read the documents.
The moving document reading will be described with reference to FIG. 14. In the moving document reading, documents laid on a document input tray 201 are separated one by one and fed (conveyed) to a reading position 202 set on a platen glass (contact glass) 203 of the reading apparatus. On an opposite side to the reading position 202 with respect to the platen glass 203, a carriage 200 holding an illumination unit 204, folding mirrors 205, an imaging optical system 206, a linear image sensor 207 and others is disposed in a standby state.
The document fed to the reading position 202 is illuminated by the illumination unit 204, and a line image of the document is projected on the linear image sensor 207 through she imaging optical system 206. The document is fed so as to continuously pass the reading position 202, and thereby the line images of respective parts of the document are continuously read by the linear image sensor 207. Line image data of the respective parts of the document from the linear image sensor 207 are sent to an image processor (not shown), and a two-dimensional image is produced thereby.
As shown in FIG. 14, there is a case where contaminants are adhered to front and rear surfaces of each document laid on the document input tray 201, and there is a case where a metal material such as a staple is attached to the document. The contaminants include a rubber eraser scrap, a paste, a white-out liquid or the like. The contaminants adhered to the front surface of the document may be rubbed against the platen glass 203 and a back plate 209 at the reading position 202 during the document feeding and be transferred to the platen glass 203 and the back plate 209 from the document to remain thereon. Moreover, if the document to which the metal material is attached is moved, the platen glass 203 and the back plate 209 may be scratched.
Furthermore, there is a case where, regardless of the document laid on the document input tray 201, contaminants are attracted from outside to be fixedly adhered to the reading position 202 due to static electricity or the like. What kind of image defect is generated due to such contaminants or scratches at the reading position 202 will be described with reference to FIG. 15B.
FIG. 15B shows three characters “can” as a two-dimensional image obtained through the image reading apparatus. The two-dimensional image is obtained by arranging line images, each extending in a main (horizontal) scanning direction, in a sub scanning direction. However, a vertical line is overlapped with the character “c.” This is because a vertical black streak image is generated in the sub scanning direction due to a black contaminant adhered to the reading position when the document is moved in the sub scanning direction.
Although part of the character “a” is broken in FIG. 15B, this is caused by a scratch generated at the reading position. As well as the contaminant, the scratch fixed at a certain horizontal scanning position diffusely reflects light to cause a vertical white streak image in a black image area in the document where reflected light is originally little, and thereby a broken portion is generated in a black character image. Accordingly, when performing the moving document reading, it is strongly recommended to perform cleaning on image correction by detecting the contaminant or scratch. However, as understood from FIG. 15A showing a line image at a sub scanning position A in FIG. 15B, it is difficult to determine, only from the line image, whether a black or white point image included therein is image information or information generated due to the contaminant or scratch.
Japanese Patent Laid-open No. 2005-006109 discloses a method of detecting, by providing a reflective member right over a reading position, contaminants adhered to the reading position while a document is not fed. Specifically, as shown in FIG. 16, the method obliquely illuminates a platen glass 203 and a back plate 209 through an image reading illumination unit 204 to detect contaminants on an upper face of the platen glass 203.
However, the method disclosed in Japanese Patent Laid-open No. 2005-006109 may erroneously detect a foreign object (including an adhered contaminant or a scratch) on the back plate 209 as a foreign object on the upper face of the platen glass 203. The contaminant or the metal material is attached not only to the front surface of the document but also to a rear surface thereof, and the contaminant adhered to the rear surface of the document is rubbed against the back plate 209 as well as the contaminant adhered to the front surface of the document and rubbed against the upper face of the platen glass 203. Similarly, the metal material scratches the back plate 209. In the method disclosed in Japanese Patent Laid-open No. 2005-006109, an image sensor 207 receives light emitted from the illumination unit 204 and then reflected at the back plate 209 through an imaging optical system 206 to detect the foreign object, which makes it impossible to determine whether the contaminant or scratch is adhered or attached to the platen glass 203 or the back plate 209.
Since the document is not influenced even when the contaminant or scratch adhered or attached to the back plate 209, no image defect is generated. However, erroneous detection of the contaminant or scratch adhered or attached to the back plate 209 causes unnecessary image correction, which may generate another image defect. Furthermore, in a case where the image reading apparatus notifies a possibility of the image defect to a user, notifications due to the erroneous detection of the contaminant or scratch may bring the image reading apparatus to a halt and thereby deteriorate productivity.